Hello, everybody,
I have a UPS with two identical fans connected in parallel. Because they
are loud, I want to replace them.
The nominal voltage of all fans is 12V.
Each of the old fans draws max 2.5W.
Each of the new fans draws max 0.96W.
If I only connect the new fans, the UPS reports a serious error (beeps
unhappily).
If I add one of the old fans in parallel to the two new fans, the error
goes away.
I think this is because the UPS recognizes a power consumption of only
about 2W instead of 5W.
I don't think that a temperature sensor triggers the error, because the
UPS works fine with two new fans and one old fan (all connected in
parallel), even if the old fan is physically outside of the case and
doesn't cool anything.
I consider connecting a resistor in parallel to the new fans.
If this is a good idea, the question is how big this resistor should be.
If not, how can I get the new fans to run wihtout triggering errors?
An old fan has a measured resistance of 650 Ohm or 1320 Ohm, depending
on the polarity.
Thank you very much.

How many wires are connected to the UPS from the fans? Measuring current
for detecting fan errors is fairly unusual in the PC space, even for
enterprise gear. In most applications you will find at least a third
wire, which indicates the fan's rotational speed back to the device.
Putting a resistor in parallel is unlikely to work, as the resistance of
the motor changes when it spins.

Riello Dialog Plus Rack 150. This is an old online UPS.
The old fans have 2 wires. The plug on the PCB has, of course, only two
wires. The new fans have 3 wires. I am using only two, obviously.
Old fan type: Sunon KDE1208PTB1
New fan type: be quiet Silent Wings 2 (BL060)
Without load the fan voltage is about 8V. I think it gradually increases
once things start getting hot and I am assuming fan speed is regulated
only by voltage, since there are only two wires.